Continuous ground-based observations provide a unique perspective on cloud properties. The Raman lidar system located at the ARM site in Oklahoma provides simultaneous observations of water vapor mixing ratio and cirrus crystal backscatter and extinction. Mixing ratio values are converted to relative humidity using temperature profiles measured at the site. We have analyzed one year of these measurements and find a significant number of occurrences where the RH with respect to ice exceeds 120% in the presence of ice. Backscatter lidar and radar observations at Nauru in the western Pacific show that high cirrus (bases over 10 km) occurs some 50% of the time and that the very highest cirrus is the product of large-scale dynamics rather than convection. These observations raise interesting questions about the life cycle of cirrus and the ability of ice formation to dehydrate near-tropopause air.

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